Plastic and nonmetallic article shaping or treating: processes – Direct application of electrical or wave energy to work – Limited to treatment of surface or coated surface
Reexamination Certificate
1999-02-17
2001-11-27
Jenkins, Daniel (Department: 1742)
Plastic and nonmetallic article shaping or treating: processes
Direct application of electrical or wave energy to work
Limited to treatment of surface or coated surface
C264S447000
Reexamination Certificate
active
06322744
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to electrochemical cells and batteries, and more particularly, to improved electrode active material of such batteries, and novel methods of synthesis.
BACKGROUND OF THE INVENTION
Lithium batteries are prepared from one or more lithium electrochemical cells containing electrochemically active (electroactive) materials. Such cells typically include an anode (negative electrode), a cathode (positive electrode), and an electrolyte interposed between spaced apart positive and negative electrodes. Batteries with anodes of metallic lithium and containing metal chalcogenide cathode active material are known. The electrolyte typically comprises a salt of lithium dissolved in one or more solvents, typically nonaqueous (aprotic) organic solvents. Other electrolytes are solid electrolytes typically called polymeric matrixes that contain an ionic conductive medium, typically a metallic powder or salt, in combination with a polymer that itself may be ionically conductive which is electrically insulating. By convention, during discharge of the cell, the negative electrode of the cell is defined as the anode. Cells having a metallic lithium anode and metal chalcogenide cathode are charged in an initial condition. During discharge, lithium ions from the metallic anode pass through the liquid electrolyte to the electrochemical active (electroactive) material of the cathode whereupon they release electrical energy to and external circuit.
It has recently been suggested to replace the lithium metal anode with an intercalation anode, such as a lithium metal chalcogenide or lithium metal oxide. Carbon anodes, such as coke and graphite, are also intercalation materials. Such negative electrodes are used with lithium-containing intercalation cathodes, in order to form an electroactive couple in a cell. Such cells, in an initial condition, are not charged. In order to be used to deliver electrochemical energy, such cells must be charged in order to transfer lithium to the anode from the lithium-containing cathode. During discharge the lithium is transferred from the anode back to the cathode. During a subsequent recharge, the lithium is transferred back to the anode where it reintercalates. Upon subsequent charge and discharge, the lithium ions (Li
+
) are transported between the electrodes. Such rechargeable batteries, having no free metallic species are called rechargeable ion batteries or rocking chair batteries. See U.S. Pat. Nos. 5,418,090; 4,464,447; 4,194,062; and 5,130,211.
Preferred positive electrode active materials include LiCoO
2
, LiMn
2
O
4
, and LiNiO
2
. The cobalt compounds are relatively expensive and the nickel compounds are difficult to synthesize. A relatively economical positive electrode is LiMn
2
O
4
, for which methods of synthesis are known, and involve reacting generally stoichiometric quantities of a lithium-containing compound and a manganese containing compound. The lithium cobalt oxide (LiCoO
2
), the lithium manganese oxide (LiMn
2
O
4
), and the lithium nickel oxide (LiNiO
2
) all have a common disadvantage in that the charge capacity of a cell comprising such cathodes suffers a significant loss in capacity. That is, the initial capacity available (amp hours/gram) from LiMn
2
O
4
, LiNiO
2
, and LiCoO
2
is less than the theoretical capacity because less than 1 atomic unit of lithium engages in the electrochemical reaction. Such an initial capacity value is significantly diminished during the first cycle operation and such capacity further diminishes on every successive cycle of operation. The specific capacity for LiMn
2
O
4
is at best 148 milliamp hours per gram. As described by those skilled in the field, the best that one might hope for is a reversible capacity of the order of 110 to 120 milliamp hours per gram. Obviously, there is a tremendous difference between the theoretical capacity (assuming all lithium is extracted from LiMn
2
O
4
) and the actual capacity when only 0.8 atomic units of lithium are extracted as observed during operation of a cell. For LiNiO
2
and LiCoO
2
only about 0.5 atomic units of lithium is reversibly cycled during cell operation. Many attempts have been made to reduce capacity fading, for example, as described in U.S. Pat. No. 4,828,834 by Nagaura et al. However, the presently known and commonly used, alkali transition metal oxide compounds suffer from relatively low capacity. Therefore, there remains the difficulty of obtaining a lithium-containing chalcogenide electrode material having acceptable capacity without disadvantage of significant capacity loss when used in a cell.
Capacity fading is well known and is calculated according to the equation given below. The equation is used to calculate the first cycle capacity loss. This same equation is also used to calculate subsequent progressive capacity loss during subsequent cycling relative back to the first cycle capacity charge reference.
(
(
FC
charge
capacity
)
-
(
FC
discharge
capacity
)
)
×
100
%
FC
charge
capacity
In U.S. Pat. No. 4,828,834 Nagaura et al. attempted to reduce capacity fading by sintering precursor lithium salt and MnO
2
materials and thereby forming an LiMn
2
O
4
intercalation compound. However, Nagaura's LiMn
2
O
4
compounds were not fully crystallized spinel electrodes and suffered from a very low capacity. Despite the above approaches, there remains the difficulty of obtaining lithium manganese oxide based electrode materials having the attractive capacity of the basic spinel Li
x
Mn
2
O
4
intercalation compound, but without its disadvantage of significant capacity loss on progressive cycling.
SUMMARY OF THE INVENTION
The present invention provides a composition suitable for use as an electrochemically active material for an electrochemical cell. The composition comprises particles of spinel lithium manganese oxide having on the surface of the particles ionic metal species bound to the spinel at oppositely charged respective ionic sites of the spinel particle surface. The ionic metal species preferably includes a transition metal. Alternatively, the ionic metal species includes a non-transition metal capable of a +3 valence state. The ionic species may contain mixtures of the foregoing metals. Cationic metal species bound to the spinel particle surface include, but are not limited to, metal cation, metal oxide cation, and metal phosphate cation.
In a preferred method, the composition comprising the spinel lithium manganese oxide having ionic species bound thereto is prepared by decomposing or melting a precursor metal compound on the surface of the spinel particles, thereby giving rise to the cationic metal species.
The spinel lithium manganese oxide treated for improved results by the method of the invention is known to have the nominal formula Li
1
Mn
2
O
4
. Such spinel lithium manganese oxide compounds may vary in the relative proportion of lithium, manganese, and oxygen while maintaining identity as a spinel lithium manganese oxide insertion compound. The invention is not limited to any particular formulation for a spinel lithium manganese oxide. However, advantageous results are obtained when the spinel lithium manganese oxide is represented by the nominal formula Li
1+x
Mn
2−x
O
4
where x is a range of about −0.2 to about +0.5; and more preferably where x is greater than zero and up to about 0.5.
The treated spinel lithium manganese oxide is prepared as an electrode by mixing it with a binder and optionally with an electrically conductive material and forming it into an electrical structure.
The composite spinel manganese oxide particles having the metal species bound thereto is prepared by first forming a mixture comprising the lithium manganese oxide particles and the metal compound. The mixture may be formed by mixing lithium manganese oxide powder and metal compound powder. Alternately, the metal compound (metal salt) is dissolved in
Kelley Tracy E.
Mitchell Porter H.
Scordilis-Kelley Chariclea A.
Harness & Dickey & Pierce P.L.C.
Jenkins Daniel
Valence Technology Inc.
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